The present invention relates to a sodium-sulfur battery having a high reliability, which is suitable for composing battery systems such as power storage equipment, electric vehicles, emergency power supplies, uninterruptible power supplies, peak shift apparatus for electric power systems, frequency-voltage stabilizers, and others, and to a battery system using same.
A sodium-sulfur battery using sodium for the negative electrode and sulfur for the positive electrode active material receives a widespread attention, because of its preferable efficiency and a large energy density, and being expected to be useful for power storage systems, electric vehicles, and others. However, the sodium-sulfur battery has a problem that corrosive sulfur and sodium polysulfide cause corrosion of a cell container for positive electrode to deteriorate the characteristics of the battery, which becomes a barrier to be overcome in practical use. That is, the sodium-sulfur battery had a problem that, when surface of the metallic vessel composing the cell container for positive. electrode, which is made of metal such as stainless steel, is corroded by sulfur and sodium polysulfide. Sulfur, which is a positive electrode active material, is consumed for forming corrosion products, and the amount of the positive electrode active material necessary for the battery reaction is decreased to lower the battery capacitance. Further, there is another problem such that an effect of electrical resistance of metallic sulfide generated at the surface of the positive electrode increases an internal resistance of the battery to lower the efficiency of the battery. In order to solve the problem, various methods, wherein the inner wall of the cell container for positive electrode is coated with a corrosion resistant coating agent composed mainly of Cr, Mo, Ti, Al, C and so on, have been disclosed. However, because of peeling by thermal cycles and defects in the coating layer, reliability of the coating is not sufficient in comparison with a case when a bulk material is used for the cell container for positive electrode. For instance, JP-A-2-142065 (1990) discloses a cell container for positive electrode made of aluminum alloy, for instance, of which surface is coated with a cobalt base alloy film containing 20.about.40 wt. % Cr, 1.about.3 wt. % C, and others by a plasma spraving method. The above case, wherein surface of the cell container for positive electrode was coated with a corrosion resistant Co base alloy film, had problems such that the adhesion and durability of the coating film fluctuated readily because the manufacturing method was complex, and reliability was not sufficient because sometimes the coating film was peeled off during assembling the battery and operation. The coating alloy layer formed by the plasma spraying method readily absorbs gases generated from the molten metal, because the coating film is formed by solidification of the molten metal. The alloy layer has a danger to cause readily swelling or peeling off by partial pressure of the gases with a temperature rise during operation of the battery. Once the swelling or the peeling occurs, the cell container for positive electrode made of an aluminum alloy forms an insulating film by contacting with molten sodium polysulfide, and such a problem occurs that an efficiency of current collection from the cell container for positive electrode decreases. Further, when an aluminum alloy is used for fabricating the cell container for positive electrode, the carbon contained in the Co base ally layer formed by the plasma spraying method reacts with aluminum base material of the positive electrode with the heat at the plasma spraying to generate aluminum carbide (Al.sub.4 C.sub.3). The carbide reacts with water in atmosphere to generate methane by a reaction shown with the following equation: EQU Al.sub.4 C.sub.3 +12H.sub.2 O=4Al(OH).sub.3 +3CH.sub.4
Therefore, handling of the cell container for positive electrode in atmosphere causes peeling off and deteriorating the alloy layer. Accordingly, consideration, such that an operation for assembling the cell container for positive electrode into the battery must be performed in an inert atmosphere, becomes necessary, and it is deemed as a disadvantage in view of mass production.
Once Al.sub.4 C.sub.3 is generated, the alloy layer has a danger to cause peeling off by a stress generated by assembling or operating the battery because of brittelness of Al.sub.4 C.sub.3. As examples of using a bulk metallic material for the cell container for positive electrode, various cases using Fe alloys containing a large amount of Cr were disclosed, for instance, in JP-A-59-165378 (1984), JP-A-57-57861 (1982), and JP-A-56-130071 (1981). In preparing a cell container for positive electrode with the above metallic material, welding is the most preferable method for finally sealing the cell container for positive electrode in reliability and operability. However, in accordance with welding by the prior art, the corrosion resistance of the material at the welding portion became worse than the bulk material. Accordingly, there was a problem that actual corrosion velocity of the cell container for positive electrode by sulfur and sodium polysulfide became faster than a value expected from a result of experimental corrosion test. As the result, a problem in reliability such as insufficient reliability of batteries, and in difficulty of life estimation because of change in corrosion resistant property depending on variation in welding conditions has been remained to be solved. Examples using welding for Fe group cell container for positive electrode were disclosed in JP-A-2-144858 (1990), JP-A-61-10881 (1986), and JP-A-48-43129 (1981). An corrosion resistance coating material was used for the cell container for positive electrode disclosed in JP-A-2-144858 (1990). However, the corrosion resistant property of the material at the welding portion decreased significantly by melting of the coating layer by the welding, and a low reliability of the battery was a problem to be solved. On the other hand, JP-A-61-10881 (1986) disclosed a positive electrode lid made of stainless steel, Fe--Cr--Al alloy, or Fe coated with Al, and a positive electrode supplementary lid made of stainless steel, Fe--Cr--Al alloy, or Fe--Cr--Al--Y alloy. However, the reference did not teach any material for the battery vessel, which was a key component of the cell container for positive electrode, nor any content of Cr and C of the Fe alloy composing the positive electrode lid, which was another key component of the cell container for positive electrode, and the positive electrode supplementary lid. In accordance with JP-A-48-43129 (1973), SUS 304 (Cr 18.about.20 wt. %, Ni 8.about.10.5 wt. %, Fe balance) was used as a material for the cell container for positive electrode. Although SUS 304 has a preferable weldability, a sufficient reliability can not be obtained because of poor corrosion resistant property against sulfur. The Fe alloy has a larger residual strain at the welding portion, and a smaller corrosion resistance in comparison with Co base alloy and Ni base alloy, and a specific resistivity of ferrous sulfide, which is a corrosion product of the Fe alloy, is higher than that of cobalt sulfide and nickel sulfide, and battery resistance readily increases. Therefore, in order to fabricate a reliable cell container for positive electrode by a welding method, composition of the Fe alloy used in the fabrication must be restricted exactly to a suitable range. However, the prior art did not consider the above restriction exactly.